1. Field of the Invention
The present invention relates to a laser-diode-excited solid-state laser apparatus in which a solid-state laser crystal doped with a rare-earth ion is excited with a laser diode (semiconductor laser) so as to emit a laser beam.
The present invention also relates to a laser-diode-excited solid-state laser apparatus in which a solid-state laser crystal doped with a rare-earth ion is excited with a laser diode (semiconductor laser), and which is arranged to emit ultraviolet light.
The present invention further relates to a laser-diode-excited fiber laser apparatus in which a core of an optical fiber doped with a rare-earth ion is excited with a laser diode (semiconductor laser) so as to emit a laser beam.
The present invention furthermore relates to a laser-diode-excited fiber laser amplifier in which a core of an optical fiber doped with a rare-earth ion is excited with a laser diode (semiconductor laser) so as to amplify incident light by utilizing fluorescence generated by the excitation of the core.
2. Description of the Related Art
(1) Solid-State Laser
Gas-laser-excited solid-state laser apparatuses in which a Pr3+-doped solid-state laser crystal is excited with a gas laser such as an Ar laser are known as disclosed in Journal of Applied Physics, vol. 48, No. 2, pp. 650–653 (1977) and Applied Physics, B58, pp. 149–151 (1994). These solid-state laser apparatuses can generate a laser beam in a blue wavelength range of 470 to 490 nm by a transition from 3P0 to 3H4 and a laser beam in a green wavelength range of 520 to 550 nm by a transition from 3P1 to 3H5. Therefore, the above solid-state laser apparatuses can be used as light sources for recording a color image in a color sensitive material.
In addition, another solid-state laser apparatus which emits a laser beam having a wavelength in the blue or green wavelength range is known. For example, the Japanese Unexamined Patent Publication No. 4(1992)-318988 corresponding to the Japanese Patent Application No. 3(1991)-086405, which is assigned to the present assignee, discloses a laser-diode-excited solid-state laser apparatus in which a solid-state laser beam is converted into a second harmonic, i.e., the wavelength of the solid-state laser beam is reduced, by arranging a nonlinear optical crystal in a resonator.
Further, InGaN-based compound laser diodes and ZnMgSSe-based compound laser diodes which emit laser beams in the blue and green wavelength ranges have recently been developed.
However, the light sources for use in recording a color image in a color image recording apparatus are required to be small in size, light in weight, and inexpensive. Nevertheless, the above gas-laser-excited solid-state laser apparatus using the Pr3+-doped solid-state laser crystal is not suitable for use in recording a color image in a color image recording apparatus since the gas laser in the gas-laser-excited solid-state laser apparatus are large, heavy, and expensive.
On the other hand, since the efficiency of wavelength conversion in the conventional laser-diode-excited solid-state laser apparatuses in which a wavelength of a solid-state laser beam is reduced by using a nonlinear optical crystal is not sufficiently high, it is difficult to obtain high output power. In addition, in such laser-diode-excited solid-state laser apparatuses, an etalon or the like is inserted for limiting the oscillation mode to a single mode. Therefore, loss in the resonator is great, and thus achievement of high output power becomes more difficult.
Further, in order to match phases in the wavelength conversion in the above laser-diode-excited solid-state laser apparatuses, highly accurate temperature control is required, and therefore the outputs of the laser-diode-excited solid-state laser apparatuses are not stable. Furthermore, the numbers of parts are increased by the provision of the nonlinear optical crystal and the etalon. Therefore, the laser-diode-excited solid-state laser apparatuses become expensive.
When InGaN-based compound laser diodes are used, the oscillation wavelengths of the InGaN-based compound laser diodes increase with increase in the indium contents, and theoretically it is possible to obtain laser beams in the blue wavelength range of 470 to 490 nm or in the green wavelength range of 520 to 550 nm. However, since the quality of the crystal deteriorates with the increase in the indium content, it is practically impossible to sufficiently increase the indium content, and the upper limit of the lengthened wavelength is about 450 nm.
In addition, blue light can be obtained by other laser diodes having an active layer made of an InGaNAs or GaNAs material. The oscillation wavelengths in these laser diodes can also be increased by doping the active layer with arsenic. However, since the quality of the crystal also deteriorates with the increase in the arsenic content, the upper limit of the wavelength realizing high output power is about 450 to 460 nm.
Further, the conventional ZnMgSSe-based compound laser diodes cannot continuously oscillate at wavelengths below 500 nm at room temperature, and the lifetimes of the conventional ZnMgSSe-based compound laser diodes are at most about a hundred hours.
In order to solve the above problems, the copending, commonly-assigned U.S. Pat. No. 6,125,132 and the Japanese Unexamined Patent Publication No. 11(1999)-17266 disclose a laser-diode-excited solid-state laser apparatus which is inexpensive, and can emit a laser beam in the blue or green wavelength range with high efficiency, high output power, and high output stability. In this laser-diode-excited solid-state laser apparatus, a Pr3+-doped solid-state laser crystal is excited with a GaN-based compound laser diode.
(2) Ultraviolet Laser
Highly efficient, high output power ultraviolet lasers which continuously oscillate in the ultraviolet wavelength range are required, for example, for applications in ultraviolet lithography, fluorometric analysis of organic cells using laser excitation, and the like.
GaN-based compound semiconductor lasers having an active layer made of an InGaN, InGaNAs, or GaNAs material are known as lasers which oscillate in the ultraviolet wavelength range. Recently, GaN-based compound semiconductor lasers which can continuously oscillate for a thousand hours at the wavelength of 400 nm with output power of several milliwatts have been provided.
On the other hand, wavelength-conversion solid-state lasers which output ultraviolet laser beams having wavelengths of 400 nm or below are known. In these wavelength-conversion solid-state lasers, wavelengths of laser oscillation light are shortened to the ultraviolet wavelengths by second harmonic generation (SHG) or third harmonic generation (THG) using nonlinear optical crystals.
However, the conventional GaN-based compound semiconductor lasers cannot emit laser light with output power of 100 mW or more in a single transverse mode, although such laser light is required in many applications. In addition, the oscillation efficiency in the conventional GaN-based compound semiconductor lasers which emit laser light having wavelengths of 380 nm or below is low, and the lifetimes of such GaN-based compound semiconductor lasers are very short.
On the other hand, wavelength-conversion solid-state lasers which output ultraviolet laser beams having wavelengths of 400 nm or below are known. In these wavelength-conversion solid-state lasers, wavelengths of laser oscillation light are shortened to the ultraviolet wavelengths by second harmonic generation (SHG) or third harmonic generation (THG) using nonlinear optical crystals.
However, solid-state laser mediums which realize efficient oscillation in the wavelength range of 700 to 800 nm have not yet been found. Therefore, it is difficult to obtain ultraviolet laser beams with high output power from the wavelength-conversion solid-state lasers in which the wavelengths of the laser light are shortened by second harmonic generation (SHG).
In addition, the efficiency of the wavelength-conversion solid-state lasers in which the wavelengths of the laser light are shortened by third harmonic generation (THG) is essentially low, and the conventional THG wavelength-conversion solid-state lasers can oscillate in only a pulse mode. In order to realize continuous oscillation, i.e., in order to maintain resonance of THG light of the fundamental wave, highly accurate temperature adjustment of a resonator with a precision of 0.01° C. is required. However, such accurate temperature adjustment is practically difficult in terms of cost.
In order to solve the above problems, the copending, commonly-assigned U.S. Ser. No. 09/621,241 and the Japanese Unexamined Patent Publication No. 2001-36175 disclose a laser-diode-excited solid-state laser apparatus in which a solid-state laser beam is converted into a second harmonic by using an optical wavelength conversion element so that ultraviolet light is obtained.
The above laser-diode-excited solid-state laser apparatus comprises: a solid-state laser crystal which is doped with at least one rare-earth ion including at least Pr3+; a laser diode which has an active layer made of one of InGaN, InGaNAs, and GaNAs materials, and emits an excitation laser beam for exciting the solid-state laser crystal; and an optical wavelength conversion element which performs wavelength conversion on a solid-state laser beam generated by excitation of the solid-state laser crystal so as to generate ultraviolet laser light.
Although the laser-diode-excited solid-state laser apparatus disclosed in the U.S. Ser. No. 09/621,241 and the Japanese Unexamined Patent Publication No. 2001-36175 can solve the aforementioned problems, the wavelength of the ultraviolet light which can be generated by the disclosed laser-diode-excited solid-state laser apparatus is limited to about 360 nm.
(3) Fiber Laser
As disclosed in the Technical Report of the Institute of Electronics, information and Communication Engineers in Japan, LQE95-30 (1995) p.30 and Optics Communications 86 (1991) p.337, laser-diode-excited fiber laser apparatuses which comprise a laser diode and an optical fiber having a core made of a Pr3+-doped fluoride are known. In the laser-diode-excited fiber laser apparatuses, the optical fiber is excited with the laser diode so as to generate a laser beam.
In addition, as disclosed in the above references, optical fiber amplifiers which comprise a laser diode and an optical fiber having a Pr3+-doped core are also known. In these optical fiber amplifiers, the optical fiber is excited with the laser diode so that fluorescent light is generated by the excitation of the optical fiber, and incident light of the optical fiber is amplified by the energy of the fluorescent light when the wavelength of the incident light is included in the wavelength range of the fluorescent light.
Further, Optics Communications 86 (1991) p.337 discloses an Ar-laser-excited, Pr3+-doped fiber laser apparatuses, and laser oscillations at the wavelengths of 491, 520, 605, and 635 nm using excitation light having a wavelength of 476.5 nm have been reported.
The above laser-diode-excited fiber laser apparatuses and Ar-laser-excited, Pr3+-doped fiber laser apparatuses can emit blue or green laser beams, and the above optical fiber amplifiers can amplify blue or green laser beams. In this respect, it is considered that these apparatuses and amplifiers can be used as constituents of light sources for recording a color image in a color sensitive material.
However, in order to operate the above Ar-laser-excited, Pr3+-doped fiber laser apparatuses or Pr3+-doped fiber laser amplifiers with high power of a few watts to several tens of watts, for example, for recording a color image, a water cooling system is necessary. Therefore, the size is increased, and the lifetime and the efficiency are reduced.
In order to solve the above problems, the copending, commonly-assigned U.S. Pat. No. 6,125,132 and the Japanese Unexamined Patent Publication No. 11(1999)-204862 disclose a fiber laser apparatus which can efficiently emit a laser beam in a blue or green wavelength range with high output power and high stability in the output and beam quality, and can be formed in a small size. In this fiber laser apparatus, an optical fiber having a core doped with Pr3+ is excited with a GaN-based compound laser diode.
In addition, the U.S. Pat. No. 6,125,132 and the Japanese Unexamined Patent Publication No. 11(1999)-204862 also disclose a fiber laser amplifier which can efficiently amplify a laser beam in a blue or green wavelength range with high output power and high stability in the output and beam quality, and can be formed in a small size. In this fiber laser amplifier, an optical fiber having a core doped with Pr3+ is excited with a GaN-based compound laser diode so as to amplify incident light of the optical fiber when the wavelength of the incident light is in the wavelength range of fluorescent light generated by the excitation of the optical fiber.
Further, the U.S. Ser. No. 09/621,241 and the Japanese Unexamined Patent Publication No. 2001-36168 disclose a fiber laser apparatus in which an optical fiber having a core codoped with Pr3+ and at least one of Er3+, Ho3+, Dy3+, Eu3+, Sm3+, Pm3+, and Nd3+ is excited with a GaN-based compound laser diode. The U.S. Ser. No. 09/621,241 and the Japanese Unexamined Patent Publication No. 2001-36168 also disclose a fiber laser amplifier in which an optical fiber having a core codoped with Pr3+ and at least one of Er3+, Ho3+, Dy3+, Eu3+, Sm3+, Pm3+, and Nd3+ is excited with a GaN-based compound laser diode so as to amplify incident light of the optical fiber when the wavelength of the incident light is in the wavelength range of fluorescent light generated by the excitation of the optical fiber.